TT farmhouse with selective RCD tripping
A 1970s farmhouse is supplied by overhead line with a TT earthing arrangement. The main earth terminal is connected to a single 16 mm² copper rod electrode. A fall-of-potential test gives 180 Ω. The installation has a 100 mA Type S RCD as the main switch, six 30 mA RCBOs on final circuits, and a 30 mA RCD protecting a socket in a detached shed. The client reports that the main 100 mA device trips occasionally at night, taking out the whole house, while the individual RCBOs remain closed. Insulation-resistance tests on all final circuits are above 200 MΩ. There is a new inverter-driven heat pump, a workshop with a 3-phase welder, and a borehole pump. A clamp meter on the main earthing conductor shows 12 mA leakage with all loads on. The shed socket RCD trips when the shed light is switched off.
1. Calculate the maximum earth-electrode resistance permitted for the 30 mA and 100 mA RCDs using the 50 V touch-voltage limit. Is 180 Ω acceptable?
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For an RCD used for fault protection in a TT system, RA × IΔn ≤ 50 V, so RA ≤ 50/IΔn. For 30 mA: 50/0.03 = 1667 Ω. For 100 mA: 50/0.1 = 500 Ω. The measured 180 Ω is below both, so it is acceptable for those two devices. At 30 mA the touch voltage would be 180 × 0.03 = 5.4 V; at 100 mA it would be 18 V. This does not mean the electrode is adequate for all devices: a 300 mA RCD would require RA ≤ 167 Ω, so 180 Ω would fail. The type and rating of each RCD must be checked, and the electrode resistance should be re-measured during the dry season because it can rise.
2. Explain why the 100 mA Type S RCD may trip while the 30 mA RCBOs do not, and what you would investigate.
Read the worked answer
A Type S device is time-delayed so that a downstream 30 mA device should trip first. If the 100 mA trips and the RCBOs hold, the fault is likely upstream of the RCBOs, or the current is not passing through any one RCBO. Possibilities include: a neutral-to-earth fault on the submain between the main RCD and the RCBOs; cumulative leakage from several circuits, especially the heat pump, welder and borehole pump, that exceeds the main RCD's operating range; a shared or borrowed neutral; a surge or lightning-induced current; or a faulty Type S device. The 12 mA clamp reading is an average and does not rule out short leakage peaks. I would measure leakage on each outgoing way, insulation-test the submains L–E and N–E, check for shared neutrals, verify the Type S marking and time delay, and ramp-test the main RCD. Splitting the installation into separate RCD groups may be needed.
3. The shed RCD trips when the light is switched off. What fault patterns could cause this, and how would you test?
Read the worked answer
Tripping on switch-off points to a transient or a fault that is exposed when the load is disconnected. Possible causes: a neutral-to-earth fault in the luminaire or its wiring; a switched-neutral arrangement leaving the fitting live; a failing LED driver or fluorescent ballast discharging to earth; capacitance in a long cable; or a borrowed neutral between the shed and another circuit. I would isolate the shed, prove dead, and insulation-test the light circuit L–E and N–E, then test the luminaire separately. I would check the switch polarity to confirm the line is switched, not the neutral. I would also ramp-test the RCD and check for shared neutrals. Replacing the luminaire or driver may be needed, but the N–E fault must be found first.